Semiconductor assembly and method of forming same



Feb. 14, 1961 w. E. TAYLOR 2,971,869

SEMICONDUCTOR ASSEMBLY AND METHOD OF' FORMING SAME Original Filed Feb. l0, 1954 /ND/UH GERAN/UM ALLOY '/v'- TYPE J2 '72'- TyPE AREA 0f cwysmz,

METAL SUCH AREA 0F CRXfAL A5 /CND/UM N'- TYPE f/wc'o/vurrof? CRYSTAL 5am A5 GERA/wan MEM ALLOY "P" TYPE parameter. In accordance with the present invention, the alloying metal is fused to faces of the crystal parallel to these planes to provide cavities and corresponding p-n junctions` that are flat rather than curved-in shape.

It is, accordinglj/,an object of the present invention to Yprovide an improved transistor unit constructed to exhibit an improved highrfrequency. response characteristicV with respectto similar units previously known to theart. Y

' Another object of thej` invention is to provide an improved and simplified process for producing junctions in a transistor crystal that are substantially flat and lie in essentially parallel planes so as to improve the high frequency response characteristic ofthe transistor unit.

` AY feature ofi the invention is the fusing of alloying metals on faces of a semi-conductor crystal wafer, which facesare essentially parallelY to the Miller Indices (111) crystallographic planesof the semi-conductor crystal.

The above yand other features of the invention which are believed to be new are set forth with particularity withwappended claims.` The invention itself, however, together with further objects and advantages thereof had best be understood kbyV reference to the following description when taken in conjunction with the accompanying drawing'in which: Y

',Fig. `l is a` schematic representation of au alloying metal fused to a nface of a Y'semi-conductor crystal in accordance with the prior art practice;

Fig. 2 is a schematic representation of such a metal fused to a face of the crystal in accordance with the present invention; and

Fig. 3 shows a junction type transistor unit constructed in accordance with the present invention.

1 The present invention provides a transistor unit which comprises a semi-conductor crystal block having a pair of opposite facesv disposed essentially parallel to the crystallographic Miller VIndices '(111) planes of the crystal block. A pair of cavities is respectively formed in these opposite faces of the crystal block, and these cavities have essentially flat surfaces substantially parallel to the aforesaid (lll) crystallographic planes of the crystal block. A pair of metallic members is respectively disposed within the cavities'and fused tothe respective surfaces thereof, and an alloy area extends into the crystal block from the surface of each of the cavities and defines Ya substantially flat planar junction within the crystal -block disposed in a-plane essentially parallel to the surface of the corresponding one of the cavities.

The formation of junction type transistor units by fusing an alloying metal to the opposite faces of a semiconductor crystal wafer is, as previously pointed out, well known. For example, a p-n-p germanium-indium transistor unit may be constructed in the following manner: v

t n-type germanium wafers are cut and then surface ground and etched in a suitable etching solution to irnpart the desired electrical characteristics to the wafer. The etching also provides the proper cleanliness to the surfacek to facilitate the wetting of the crystal by the metal that is to be fused and alloyed therewith.

','The wafer is then placed in a jig composed of graphite-or other -suitable substance, and an indium pellet that is to form the collector electrode is located at the appropriate place on one face of the wafer by a graphite washer. The assembly is then tired in a furnace to melt the indium and start its alloying with the germanium. The germanium Wafer is then turned over and a second indium pellet is placed on its opposite face and the procedure is then repeated so that the latter pellet is also bonded to the germanium and subsequently forms the emitter electrode of the transistor. The assembly is then fired at an alloying temperature so that the two faces of the germanium crystal may be corrosively attacked by the indium to form cavities andl p-type areas in the manner previously described herein.- i

amuse@ With reference now to Fig. l, there is shown a prior art junction formed in a crystal wafer when no consideration is given to the orientation of the crystal. The representation of Fig. l includes a semi-conductor crystal 19 which, for example, may be an n-type germanium wafer. This wafer has a face 1l on which a disc l2 of, for example, p-type metal such as indium, is fused. The fusing of the indium causes a cavity 13 to be formed in face 11 and produces a p-type area 14 within crystal l0 conforming to the surface of the cavity. As previously described, the surface of cavity 13 normally has a curved surface to provide a minimum surface consistent with the enclosure of the existing volume. This, however, as previously described produces unequal path lengths for the individual charge carriers with resulting adverse effects on the high frequency response of the crystal.

In accordance withV the present invention, and as shown in Fig. 2, the crystal wafer 20 is cut so that its face 21 is disposed parallel to the crystallographic Miller Indices (lll) planes of the crystal structure. l

It has been found that when the crystal is so oriented,- the corrosive attack of the alloying metal 22 produces a cavity 23 having a flat surface essentially parallel to the aforesaid crystallographich planes. Moreover, the' p-type area 24 formed during the alloying process conforms with the surface of cavity 23 and forms a p-n junction with crystal 20 that is also substantially at and lies in a plane parallel to the aforesaid crystallographic planes.

It is evident that when crystal 20 is in the form of a hat wafer, and when the-alloying metal is formed on its opposite faces in the manner previously described herein, that a pair of p-n junctions is formed on opposite sides of the crystal which are essentially at and disposed in substantially parallel relation with one anotheri The transistor unit shown in Fig. 3 is constructed t0 include a crystal wafer 30 having a pair of opposing faces 31, l32 disposed essentially parallel to the crystallographic Miller Indices (lll) planes of the crystal block.- The unit includes an emitter electrode 33 and a collector' electrode 34 respectively disposed on the opposite faces and fused thereto in the manner previously described herein to form appropriate junctions within the crystal.

A metallic tab 35 is soldered to the bottom of crystal 30 and forms the base electrode. Wire leads 36, 37 and 38 respectively connect the electrodes of the crystal to pins 39, 40 and 41. Pins 39-41 may be mounted in spaced relation on an Ainsulating base 42 to extend through the base. A suitable cover 43 is provided, and the unit may be imbedded in an appropriate potting compound within the cover. f Y

As previously noted, the technique of this invention has also been applied in the formation of n-p-n junction transistors. f Highly favorable results have been obtained when a tin-antimony alloy (about tin and 5% antimony--byhweigh is fired on a P-type germanium crystal with the crystal oriented so that the (lll) planes are parallel to the surfaces on which the alloy is fused. Other tin ,alloys containing, Vfor example, 510% phosphorus, arsenic, bismuth or the like, mayY also be used.

i The invention provides, therefore, an improved junction transistor unit which is constructed Vso that the junctions within the transistor .crystal lie in at planes that are essentially parallel to one another. In this manner, the transit time dispersion due to unequal path lengths for the individual carriers is minimized,and the adverse effect of transit time' dispersion on the high frequency characteristics of theA transistor is substantially reduced.

While a particular embodiment of the invention has been shown as described, modifications may be made, and itis intended in .theiappended claims to cover all such modifications as fall within the truespirit and scope,

of the invention. Y

I claim:

1. The method of fabricating an alloyed junction semi` conductor assembly so as to provide at least one junction in a semiconductor crystal element in such assembly which junction is substantially parallel to a predetermined crystallographic 111) plane within such element, which method includes the steps of preparing a semiconductor crystal element having at least one face which is known to be oriented substantially parallel to a (lll) crystallographic plane within the element, providing an impurity body at said face and in contact therewith, alloying said impurity body to said element at said face by subjecting said element and said' impurity body to heat of a temperature suchV that said impurity body and the portion of the semiconductor crystal element adjacent thereto alloy to one another to form a junction therein with said semiconductor crystal element, said junction being substantially planar in configuration over at least a' portion thereof and with said latter portionbeing substantially parallel to said (111) plane as a' result of having the adjacent face of the semiconductor crystal element substantially parallel to said (111) plane within said element.

2. The method of fabricating an alloyed junction semiconductor assembly so as to provide at least two junctions in the semiconductor crystal element in such assembly which junctions are substantially parallel to a predetermined crystallographic (lll) plane within such element, which method includes the steps of preparing a semiconductor crystal element having oppositely disposed faces which are known to be oriented substantially parallel to a (lll) crystallographic plane within the element, providing an impurity body at each said face and in contact therewith, alloying said impurity bodies to said element at said faces by subjecting said element and said impurity bodies to heat of a temperature such that said impurity bodies and the portion of the semiconductor crystal element adjacent thereto alloy to one another to form a pair of junctions therein with said semiconductor crystal element, each said junction being substantially planar in configuration over at least a portion thereof and with said latter portion of each junction being substantially parallel to said (lll) plane as a result of having the corresponding face of the semiconductor crystal element oriented substantially parallel to said (lll) plane within said element and having said faces of said semiconductor crystal element substantially parallel to one another.

3. The method of fabricating an alloyed junction semiconductor assembly for use in a semiconductor device and fabricating it so as to provide at least two oppositely disposed junctions in the semiconductor crystal element in such assembly and provide such junctions so as to be substantially parallel to a predetermined crystallographic (lll) plane within such element, which method includes the steps of preparing a semiconductor crystal element having oppositely disposed faces which are known to be oriented substantially parallel to a (lll) crystallographic plane within the element, providing an impurity body at each said face and in contact therewith and oppositely disposed to one another, alloying said impurity bodies to said element at said faces by subjecting said element and said impurity bodies to heat of a temperature such that said impurity bodies and the portion of the semiconductor crystal element adjacent thereto alloy to one another to form a pair of junctions therein with said semiconductor crystal element, each said junction being substantially planar in configuration over at least a portion thereof and with said latter portion of each junction being substantially parallel to said (111) crystallographic plane as a result of having the corresponding face of the semiconductor crystal element oriented substantially parallel to said (111) plane within said element and having said faces of said semiconductor crystal element substantially parallel to one another.

4f. A semiconductor assmbiy' ef filetype' saying at? least one alloyed juiictior'r;l said assembly including in combination a semiconductor crystal element with a face thereof known to be oriented substantially parallel to ay crystallographic plane within the crystal element' identified by Miller Indices (111). at least onev electrode at saidVv face of said element which isv alloyed thereto, ani alloyed region extending into said semiconductor crystal elementik from saidl face and said electrode,A said alloyed regioni having a junction formed between the innermost portion thereof and the adjacent region within the semiconductor crystal element, said junction having a substantially liat4 portion which is substantially parallel to the (lll)l crystallographic plane within the semiconductor crystal element and which is substantially parallel to said face; n

5. A semiconductor assembly'- for a `sie'rnicunductor device, said assembly including (in combination :a settliconductor crystal element having two* opposite' faces which are each known lto b e oriented substantially parallel to a crystailographic plane within theV crystal elc-" ment identified by Miller Indices (111), at least one electrode at each said face of said element and alloyed thereto, at least two alloyed regions with each extending into said semiconductor crystal element from a corresponding face and correspondingelectrode, with each said alloyed region having a junction formed between the innermost portion thereof and the adjacent region within the semiconductor crystal element, said junctions each having a substantially iiat portion which is substantially parallel to the (111) crystallographic plane within the semiconductor crystal element and which is substantially parallel to the corresponding face, and with said substantially fiat portions of said junctions being substantially parallel to one another.

6. A semiconductor assembly in a semiconductor device of the alloyed junction type, said assembly including in combination a semiconductor crystal element having two opposite faces which were originally identified in the preparation of the element as being substantially parallel to a crystallographic plane within the crystal element designated by Miller Indices (lll) and which are each known to be so oriented, at least one electrode at each said face of said element oppositely disposed thereon and alloyed thereto, an alloyed region corresponding to each electrode and extending into said semiconductor crystal element from a corresponding face and corresponding electrode, with each said alloyed region having a junction formed between the innermost portion thereof and the adjacent region within the semiconductor crystal element, said junctions each having a substantially at portion which is substantially parallel to the (lll) crystallographic plane within the semiconductor crystal element and which is substantially parallel to the corresponding face, and with said substantially flat portions ofpsaid junctions being substantially parallel to one another.

7. The method of fabricating an alloyed junction semiconductor assembly having a semiconductor crystal element in such assembly so as to provide at least two junctions therein which are substantially parallel to a predetermined crystallograph (lll) plane within such element, which method includes the steps of separating from a semiconductor crystal a semiconductor crystal element which has oppositely disposed faces known to be oriented so as to be substantially parallel to a 111) crystallographic plane within the semiconductor crystal element, providing an impurity body at each said face, alloying said impurity bodies to said element at said faces so as to form a pair of junctions within said semiconductor crystal element which are each substantially planar in configuration over at least a portion thereof, with said latter portion of each junction being in a position within the crystal element substantially parallel to said (lll) plane, and with said positions of said junctions being the result of having the said faces of the semiconductor crystal element originally oriented substantially parallel to said (111) plane within said element and having said Vfaces of said semiconductor crystal element substantially parallel to one another.

\ 8. In a transistor, the combination including a mounting; Ya semiconductor assembly on said mounting with a semiconductor crystal element having two opposite faces which are each known to have been originally oriented so as to be substantially parallel to a crystallographic plane within the crystal element identified by Miller 1ndices (111), at least one electrode atreach said face of said element and alloyed thereto, at least two alloyed regions with each said region extending into said semiconductor crystal element from a corresponding face and corresponding electrode, with each said alloyed region having a junction formed between the innermost portion thereof and the adjacent region within the semiconductor crystal element, said junctions each having a substantially at portion which is substantially parallel to the (111) crystallographic plane within the semiconductor crystal element and which is subtantially parallel to the corresponding face from which the alloyed region therefor extends, and with said substantially at portions of said junctions being substantially parallel to one another.l

5 References Cited in the tile of this patent I' UNTTED' STATES PATENTS Y 2,505,633 Whaley Apr. 25, 195 2,697,052 Y Dacey Dec. 14, 1954 10 2,742,383 Barnes et al. Apr. 17, 1956 2,802,159 Stump Aug. 6, 1957 2,850,413 Schmich e Sept, 2, 1958 2,858,730 Hanson Nov. 4, 1958 2,894,862

Mueller June 14, 1959y OTHER REFERENCES Wynne and Goldberg: Journal of Metals, page 436,

March 1953.

Ellis: Journaliof AppliedvPhysics, December 1954, 20 pages 1497-99. Y Y 

1. THE METHOD OF FABRICATING AN ALLOYED JUNCTION SEMICONDUCTOR ASSEMBLY SO AS TO PROVIDE AT LEAST ONE JUNCTION IN A SEMICONDUCTOR CRYSTAL ELEMENT IN SUCH ASSEMBLY WHICH JUNCTION IS SUBSTANTIALLY PARALLEL TO A PREDETERMINED CRYSTALLOGRAPHIC (111) PLANE WITHIN SUCH ELEMENT, WHICH METHOD INCLUDES THE STEPS OF PREPARING A SEMICONDUCTOR CRYSTAL ELEMENT HAVING AT LEAST ONE FACE WHICH IS KNOWN TO BE ORIENTED SUBSTANTIALLY PARALLEL TO A (111) CRYSTALLOGRAPHIC PLANE WITHIN THE ELEMENT, PROVIDING AN IMPURITY BODY AT SAID FACE AND IN CONTACT THEREWITH, ALLOYING SAID IMPURITY BODY TO SAID ELEMENT AT SAID FACE BY SUBJECTING SAID WLEMENT AND SAID IMPURITY BODY TO HEAT OF A TEMPERATURE SUCH THAT SAID IMPURITY BODY AND THE PORTION OF THE SEMICONDUCTOR CRYSTAL ELEMENT ADJACENT THERETO ALLOY TO ONE ANOTHER TO FORM A JUNCTION THEREIN WITH SAID SEMICONDUCTOR CRYSTAL ELEMENT, SAID JUNCTION BEING SUBSTANTIALLY PLANAR IN CONFIGURATION OVER AT LEAST A PORTION THEREOF AND WITH SAID LATTER PORTION BEING SUBSTANTIALLY PARALLEL TO SAID (111) PLANE AS A RESULT OF HAVING THE ADJACENT FACE OF THE SEMICONDUCTOR CRYSTAL ELEMENT SUBSTANTIALLY PARALLEL TO SAID (111) PLANE WITHIN SAID ELEMENT.
 6. A SEMICONDUCTOR ASSEMBLY IN A SEMICONDUCTOR DEVICE OF THE AWLLOYED JUNCTION TYPE, SAID ASSEMBLY INDLUDING IN COMBINATION A SEMICONDUCTOR CRYSTAL ELEMENT HAVING TWO OPPOSITE FACES WHICH WERE ORIGINALLY IDENTIFIED IN THE PREPARATION OF THE ELEMENT AS BEING SUBSTANTIALLY PARALLEL TO A CRYSTALLOGRAPHIC PLANE WITHIN THE CRYSTAL ELEMENT DESIGNATED BY MILLER INDICES (111) AND WHICH ARE EACH KNOWN TO BE SO ORIENTED, AT LEAST ONE ELECTRODE AT EACH SAID FACE OF SAID ALLOYED REGION CORRESPONDING GO AND ALLOYED THERETO, AN ALLOYED REGION CORRESPONDING TO EACH ELECTRODE AND EXTENDING INTO SAID SEMICONDUCTOR CRYSTAL ELEMENT FROM A CORRESPONDING FACE AND CORRESPONDING ELECTRODE, WITH EACH SAID ALLOYED REGION HAVING A JUNCTION FORMED BETWEEN THE INNERMOST PORTION THEREOF AND THE ADJACENT REGION WITHIN THE SEMICONDUCTOY CRYSTAL ELEMENT, SAID JUNCTIONS EACH HAVING A SUBSTANTIALLY FLAT PORTION WHICH IS SUBSTANTIALLY PARALLEL TO THE (111) CRYSTALLOGRAPHIC PLANE WITHIN THE SEMICONDUCTOR CRYSTAL ELEMENT AND WHICH IS SUBSTANTIALLY PARALLEL TO THE CORRESPONDING FACE, AND WITH SUBSTANTIALLY FLAT PORTIONS OF SAID JUNCTIONS BEING STBSTANTIALLY PARALLEL TO ONE ANOTHER. 